Conventionally, a common rail type fuel injection system (hereinafter referred to as “common rail system”) has been proposed, wherein high-pressure fuel accumulated in the common rail is injected into each cylinder in a multi-cylinder diesel engine. The common rail system performs a pilot injection control, wherein a small amount of fuel is injected several times prior to a main injection, which mainly makes torque of the engine, to stabilize combustion from the beginning of the main injection. To inject the small amount of fuel several times, an electromagnet valve of an injector is actuated several times during one combustion stroke of the engine. The pilot injection control is aimed at reducing engine vibrations and combustion noise, and improving emission from the engine.
Additionally, a technique, which strikes a balance between the pilot injection control and an injection quantity correction control for an engine speed variation between the cylinders (non-uniformity amount correction: FCCB correction), has been proposed. Furthermore, a technique, which strikes a balance among the pilot injection control, a correction in an idling speed control (ISC correction) and other correction controls, has been also proposed (for example, JP-A-7-063104).
In general, the common rail system calculates a command injection quantity adding a correction amount of an injection quantity considering engine coolant temperature, fuel temperature and so on, to a basic injection quantity, which is calculated in accordance with an engine speed and an acceleration stroke. After that, the system calculates a current supply period (command value of injection quantity) of a pulsed injector driving signal (TQ pulse) in accordance with the command injection quantity and a fuel injection pressure (common rail pressure) detected by, for example, a fuel pressure sensor.
The common rail system controls a fuel injection quantity supplying into each cylinder of the engine actuating the electromagnetic valve of each injector based on the command value of the injection quantity. Thus, the command value of the injection quantity supplied to the electromagnetic valve of the injector comes to control the fuel injection quantity in the common rail system, and usually, a relationship between the command value of the injection quantity (command injection pulse period) and an actual fuel injection quantity is assured with respect to each of the injectors.
However, the fuel injection amount assured with respect to each of the injectors has an error and the error becomes larger after a performance (function) degradation of the injector due to, for example, a secular change (secular change degradation of injector) has been occurred. Although the error of the injection quantity to the command value of the injection quantity is assured adjusting the injectors individually, the pilot injection quantity may become too small or even zero due to the error when the correction amount (ΔQ) by the conventional FCCB correction control is a negative value because the pilot injection quantity is as small as or less than 5 mm3/st. on the contrary, the pilot injection quantity may become too large when the correction amount (ΔQ) by the conventional FCCB correction control is a positive value.
In such a case, the purpose of the pilot injection control cannot be achieved enough. Especially, under a high injection pressure, the error of the injection quantity to the command value of injection quantity tends to become larger so that assuring the performance of each of the injectors is difficult in a very small injection quantity such as 1 mm3/st.
To overcome such problems, a learning control system for the pilot injection quantity has been proposed. The system learns the error of the injection quantity to the current supply period of the TQ pulse in the pilot injection control using the conventional FCCB correction or the idle speed correction (ISC correction) as described in the JP-A-07-063104. However, because the condition for learning the correction amount is limited to an injection condition (for example, an injection pressure of fuel) in a stable idling operation, the learned correction amount cannot be accurately. reflected under other fuel injection pressures mainly used during the vehicle is running or under a high injection pressure. Additionally, in a case where the system described in the JP-A-07-063104 is applied to an eight-cylinder engine, the relationship between the error and the variation is low in the eight-cylinder engine because the error of the injection quantity between cylinders and the variation of the rotation speed between cylinders hardly appear. Therefore,it is unable to precisely learn the pilot injection quantity between cylinders in such a multi-cylinder engine.